I have had pretty good luck with the Living Pathfinder games. There is some drop-out, but it seems minimal and I have had several adventures complete there. If a GM happens to abandon the game, the Living worlds have judges who can step in to take the game to completion.
My success rate with PbPs is much higher in the Living World games than random pick-up games, where I agree there is a high dropout/fail rate.
PbP can be a fun way to get some gaming in if you do not have a local group, aren't into G+ or Skype games and such.